The Global System for Mobile Communications (GSM) is a cellular telecommunications system that is used throughout the world. In one specific implementation, GSM utilizes two bands of 25 MHz, which have been set aside for system use. The 890–915 MHz band is used for subscriber to base station transmissions (reverse link), and the 935–960 MHz band is used for base station to subscriber transmissions (forward link). However, it can be appreciated that other bandwidths and frequency ranges may also be used to implement the GSM protocol. The GSM protocol uses frequency division duplexing and time division multiple access (TDMA) techniques to provide base stations with simultaneous access to multiple users. Transmissions on both the forward and reverse link are made at a channel data rate of approximately 270 Kbps, using binary Gaussian minimum shift key (GMSK) modulation.
In the GSM protocol, there are traffic channels and control channels. The traffic channels carry the digitized voice or user data. The control channels include what is known as the frequency correction channel (FCCH), which is a special data burst which occupies time slot 0 for the very first GSM frame and is repeated every ten frames within a control channel multiframe. The FCCH allows each subscriber unit to synchronize its internal frequency (local oscillator) to the exact frequency of the base station.
Typically, the frequency correction channel is a single tone with a nominal frequency within a given frequency range, and repeats every 50 bursts. However, factors such as strong Gaussian noise, strong co-channel or adjacent channel interference, and severe fading, have made a typical methods quite unreliable and time consuming. The net effect from the user's perspective thus becomes long establishing periods.
Another control channel is the synchronization control channel (SCH), which is typically broadcast in timeslot zero (TS 0) of the frame immediately following the FCCH frame and is used to identify the serving base station while allowing each mobile unit to frame synchronize with the base station. The frame number (FN) is sent with the base station identity code (BSIC) during the SCH burst. The BSIC is uniquely assigned to each base station in a GSM system.
In order to decode the SCH data burst, it is generally required to estimate the time slot timing of the SCH data burst. A rough time slot estimate is required to position the SCH data burst for decoding. More exact frame timing can be achieved after the SCH burst has been decoded by examining the channel peak acquisition. As long as the error of the rough time slot timing is within a certain limit, the handshake process between the base station and the mobile unit can be successful. However, if the error in the rough timing estimate is larger than what the SCH burst decoding process can handle, then the SCH burst decoding will fail.